The present invention relates to a process and a device allowing reinforcing elements such as fibers, fabrics, or braids to be preimpregnated before using them in the production of composite materials for the parts that are to be made.
By comparison to (continuous) impregnation of reinforcing elements, which is done just before using the composite parts to be manufactured, preimpregnation allows the product obtained to be stored for a relatively long time (several months), the number of bubbles included in the parts, to be reduced, and fraying of the reinforcing elements to be reduced.
More particularly, the present invention provides an improvement in the process and device for preimpregating reinforcing elements with a stabilizable substance such as a liquid thermosetting resin.
Previous reinforcing element preimpregnation techniques consist for example of causing the substance to penetrate under pressure, but penetration is not homogeneous and gases such as air or steam are dissolved.
The technique currently in broadest use for preimpregnation consists of causing the resin (diluted with a volatile solvent such as acetone or methanol) to penetrate reinforcing fibers by passage through a bath or over a roller, then causing this solvent to evaporate in a tunnel furnace of considerable length, and finally winding the preimpregnated thread or strip.
Usually, the heat treatment brings the resin into state B, meaning that the resin is solid at ambient temperature but can be melted by reheating.
If the resin melting point can be reached during storage, an antiadhesive film is interposed beneath the preimpregnated threads before winding, so that unwinding is always possible under cold conditions without injuring the reinforcing fibers. Without interposition of such an antiadhesive film, and if the melting point is reached during storage, the spool after cooling would turn into a solid block.
It is not possible to unwind a thread or strip from a spool that has turned into a block without at least seriously damaging the fibers, which would bring about a severe deterioration of the mechanical properties of the object eventually made with these fibers.
The presence of solvent, necessary for impregnation to be rapid with resins that are viscous even when hot, such as epoxy resins, results in the quantity of resin actually entrained by the fibers being small, on the order of 25 to 37% by volume.
It is not possible, with these prior processes, to obtain preimpregnated reinforcing elements with high resin contents--45 to 55% by volume.
For the purpose of preimpregnation with a resin diluted with a solvent, avoiding the continuous drying tunnel furnace, it has also been proposed that the spool be impregnated directly by creating a soft vacuum to prevent the solvent from boiling, then admitting the resin diluted with the solvent, and then applying a pressure to force the dilute resin into the fibers.
It is then eventually necessary to drive off the solvent by heat treatment in a soft vacuum to prevent the solvent from boiling.
This treatment brings the resin into state B, meaning that the product is virtually unusable without serious damage to the fibers.
In both cases, the use of a solvent in impregnation decreases the volume when the solvent is removed. This decrease in volume causes voids to appear between the fibers, or peeling at the glass-resin interface with the appearance of bubbles.
The products cast from these preimpregnated materials are not transparent but translucent or milky in the best case, owing to the presence of numerous microbubbles.
These bubbles considerably impair the mechanical properties which are not dependent on the presence of fibers alone. Thus, shear resistance is substantially affected by the presence of microbubbles.
These microbubbles also permit condensation of water that has penetrated by permeability through the resin, and this condensed water may attack the reinforcements which are sensitive to it, such as fiberglass. The properties may then deteriorate over time, particularly in a moist environment and in the presence of temperature cycles, corresponding to natural aging.
To avoid these drawbacks, it has been proposed that objects that have been pre-formed, for example by winding onto a mandrel, be impregnated under vacuum by solvent-free resin and subjected to vacuum treatment allowing the air to be drawn out of the fibres and the object to be impregnated to the core, giving a good-quality product after polymerization; the mandrel is then removed.
The Grummi document in Asbest, Kunststoffe, 1982, Vol. 35, pages 630-635 shows that the thicknesses impregnatable by this process are small unless the time is prohibitively long. In practice, the maximum impregnation thickness is limited to 12 mm.